This invention relates to a novel process for the preparation of organic polyisocyanates by thermal decomposition of the corresponding carbamic acid esters upon which the polyisocyanates are based.
It has long been known that N-substituted urethanes can be thermally decomposed in the gaseous or the liquid phase into isocyanates and alcohol. For example, A. W. Hofmann, Ber. Dtsch. Chem. Ges., 3, 653 (1870); and H. Schiff, Ber. Dtsch. Chem. Ges., 3, 649 (1870).
U.S. Pat. No. 2,409,712 discloses a process in which recombination of the products obtained from the solvent-free decomposition of carbamic acid esters can be prevented by introducing the products into a cyclohexane-water mixture. This process, however, provides only moderate isocyanate yields because of the partial hydrolysis of the resulting isocyanate at the phase interface.
The processes according to U.S. Pat. Nos. 3,962,302 and 3,919,278, for example, take place in the presence of inert high boiling solvents. In these processes, the two products of decomposition, that is, the alcohol and the isocyanate, are together continuously distilled from the decomposition medium and separated by fractional condensation. The disadvantages of these processes lie in the considerable technical expenditure required for the separation of the alcohol and isocyanate vapors and the moderate yields obtained. Readily volatile isocyanates are difficult to remove from the decomposition medium by distillation because of the high dilution and consequent low partial vapor pressure.
In the processes according to U.S. Pat. No. 3,919,279, German Offenlegungsschrift 2,635,490 or German Offenlegungsschrift 2,942,543, homogeneous or heterogeneous catalysts are used for increasing the volume/time yields. According to European Application 61,013, secondary isocyanate reactions are suppressed by the addition of stabilizing additives, but such additives cannot reduce the difficulties encountered in the required distillation of the isocyanates.
According to European Application 92,738, secondary isocyanate reactions may be suppressed by keeping the dwell time short. To achieve this, the molten carbamic acid ester is carried along the internal wall of a tube reactor in the presence of small quantities of an auxiliary solvent. The high boiling by-products, as well as the auxiliary solvent, are discharged as sump product, whereas the decomposition gases consisting of isocyanate and alcohol are removed at the top and separated by fractional condensation. Because only small quantities of auxiliary solvents are employed in this process, so that the carbamic acid ester is subjected to thermolysis in a virtually undiluted state, the formation of highly viscous polymeric by-products cannot be avoided. Moreover, the isocyanate removed at the top is invariably contaminated with carbamic acid esters caused by partial recombination with the alcohol that is also removed at the top.
An object of the present invention is to provide a process for the preparation of organic polyisocyanates by thermal decomposition of the carbamic acid esters corresponding to the desired polyisocyanates, whereby the polyisocyanates can be obtained without the formation of by-products and without recombination with the alcohol formed in the process. This object has been accomplished by the process of the invention described below.